UNIT – II

Hot & Cold Working - Drawing & Extrusion

MANUFACTURING TECHNOLOGY

Department of Automobile Engineering

Drawing

• Drawing is an operation in which the cross-section of solid rod, wire

or tubing is reduced or changed in shape by pulling it through a die.

Drawn rods are used for shafts, spindles, and small pistons and as

the raw material for fasteners such as rivets, bolts, screws.

• Drawing also improves strength and hardness when these

properties are to be developed by cold work and not by subsequent

heat treatment.

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Drawing Types • Wire Drawing

– Cross-section of a bar, rod, or wire is reduced by pulling it through a die opening

– Similar to extrusion except work is pulled through die in drawing (it is pushed through in extrusion)

– Although drawing applies tensile stress, compression also plays a significant role since metal is squeezed as it passes through die opening

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Drawing of wire. Department of Automobile Engineering

• Wire drawing machines consisting of multiple draw dies (typically 4 to 12) separated by accumulating drums

– Each drum (capstan) provides proper force to draw wire stock through upstream die

– Each die provides a small reduction, so desired total reduction is achieved by the series

– Annealing sometimes required between dies to relieve work hardening

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Continuous drawing of wire Department of Automobile Engineering

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Area Reduction in wire Drawing

• Change in size of work is usually given by area reduction

– where r = area reduction in drawing; Ao = original area of work; and Af = final work

Die Materials

• Commonly used materials are Tool Steels and Carbides

• Diamond dies are used for fine wire.

• For improved wear resistance, steel dies may be chromium plated, and carbide dies may be coated with titanium nitride

• For Hot drawing, cast-steel dies are used

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Bar or Rod Drawing

• Accomplished as a single-draft operation - the stock is pulled

through one die opening

• Beginning stock has large diameter and is a straight cylinder

• Requires a batch type operation

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Hydraulically operated draw bench for drawing metal bars Department of Automobile Engineering

Tube Drawing

• Accomplished by pulling the stock through the sides of the mandrel

placed between dies

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Department of Automobile Engineering

Wire Drawing vs. Bar Drawing

• Difference between bar drawing and wire drawing is stock size

– Bar drawing - large diameter bar and rod stock

– Wire drawing - small diameter stock - wire sizes down to 0.03

mm (0.001 in.) are possible

• Although the mechanics are the same, the methods, equipment, and

even terminology are different

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Preparation of Work for Drawing

• Annealing – to increase ductility of stock

• Cleaning - to prevent damage to work surface and draw die

• Pointing – to reduce diameter of starting end to allow

insertion through draw die

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Department of Automobile Engineering

Features of a Draw Die

• Entry region - funnels lubricant into the die to prevent

scoring of work and die

• Approach - cone-shaped region where drawing occurs

• Bearing surface - determines final stock size

• Back relief - exit zone - provided with a back relief angle

(half-angle) of about 30

• Die materials: tool steels or cemented carbides

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Draw Die Details

• Draw die for drawing of round rod or wire

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Department of Automobile Engineering

Extrusion

A plastic deformation process in which metal is forced under pressure

to flow through a single, or series of dies until the desired shape is

produced.

Process is similar to squeezing toothpaste out of a toothpaste tube

In general, extrusion is used to produce long parts of uniform cross

sections

Typical products made by extrusion are railings for sliding doors, tubing having carious cross-sections, structural and architectural shapes, door and windows frames.

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Department of Automobile Engineering

• Extrusion Ratio

– ER= A o /A f

– A o – cross-sectional area of the billet

– A f - cross-sectional area of extruded product

• Extrusion Force

– F = A o K Ln (A o/A f)

– K-extrusion constant

– A o , A f billet and extruded product areas

• Types

– Direct Extrusion (Forward Extrusion)

– Indirect Extrusion (Backward Extrusion)

– Hydrostatic Extrusion

– Impact Extrusion

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Department of Automobile Engineering

Direct Extrusion

• Billet is placed in a chamber and forced through a die opening by a

hydraulically-driven ram or pressing stem.

• Dies are machined to the desired cross-section

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Friction increases the extrusion force Department of Automobile Engineering

Direct Extrusion

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Schematic illustration of direct extrusion process Department of Automobile Engineering

Direct Extrusion

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Hollow and Semi Hollow section is formed using a mandrel Department of Automobile Engineering

Indirect Extrusion

• Metal is forced to flow through the die in an opposite direction to the ram’s motion.

• Lower extrusion force as the work billet metal is not moving relative to the container wall.

Limitations

• Lower rigidity of hollow ram

• Difficulty in supporting extruded product as it exits die

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Indirect extrusion to produce (a) a solid cross section and (b) a hollow cross section. Department of Automobile Engineering

Indirect Extrusion

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Schematic illustration of indirect extrusion process Department of Automobile Engineering

Process Variables in Direct & Indirect Extrusion

• The die angle

• Reduction in cross-section A f

• Extrusion speed

• Billet temperature,

• Extrusion pressure.

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Tube Extrusion

• Accomplished by forcing the stock through the sides of the mandrel placed between dies

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Tube or Pipe

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Hydrostatic Extrusion

• The pressure required for extrusion is supplied through and incompressible fluid medium surrounding the billet

• Usually carried at room temperature, typically using vegetable oils as the fluid

• Brittle materials are extruded generally by this method

• It increases ductility of the material

• It has complex nature of the tooling

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Department of Automobile Engineering

Hydrostatic Extrusion

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Schematic illustration Hydrostatic Extrusion process Department of Automobile Engineering

Manufacturing Technology Impact Extrusion

• Similar to indirect extrusion

• Punch descends rapidly on the blank, which is extruded backward

Schematic illustration of the impact-extrusion process. The extruded parts are stripped by the use of a stripper plate, because they tend to stick to the punch

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Hot extrusion

– prior heating of billet to above its recrystallization temperature

Cold extrusion

– prior heating of billet to below its recrystallization temperature

Advantages

– Wide variety of shapes

– High production rates

– Improved microstructure and physical properties

– Close tolerances are possible

– Economical

– Design flexibility

Limitation

– part cross section must be uniform throughout length

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Department of Automobile Engineering

Extrusion Die Features

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(a) Definition of die angle in direct extrusion; (b) effect of die angle on ram force. Department of Automobile Engineering

Extrusion Defects

a. Centre-burst: internal crack due to excessive tensile stress at the

centre possibly because of high die angle, low extrusion ratio.

b. Piping: sink hole at the end of billet under direct extrusion.

c. Surface cracking: High part temperature due to low extrusion speed

and high strain rates.

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Department of Automobile Engineering

Factors Influencing the Extrusion Force

• Friction

• Material Properties

• Reduction In Area

• Speed

• Temperature

• Geometry Of The Die

Manufacturing Technology

Department of Automobile Engineering